Why should adding a small amount of a stretchy material make an otherwise non-stretchy fabric stretch? Shouldn't the non-stretch fibres still constrain the maximum stretch of the fabric?
[Physics] How does a fabric containing 10% stretch material make it stretchy
elasticitymaterial-science
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Referring to your graph which is for a ductile material I suggest the following.
A is the limit of proportionality up to which the stress and strain are proportional to one another and when unloaded the material goes back to its original length.
B is the elastic limit.
With stresses below this the material behaves elastically i.e., when unloaded returns back to its original length although at the highest stress the graph is no longer a straight line.
It is also true that there is greater increase in strain for a given increase in stress in stress beyond the yield point.
C is the yield point which when reached will result in a permanent deformation of an arbitrarily defined amount of strain (sometimes $0.2\%$) when the stress is removed.
E is the onset of necking (reduction in cross sectional area of the material) where the stress is actually increasing (graph ending at D) but the evaluated stress used for plotting some graphs assumes the cross section of area of the material does not change (graph ending at F).
A really good material to observe these effects is brass in the form of a wire which is about 2 or 3 metres long.
Initially with small loads the wire extends a little bit and elastically.
Then beyond a certain load when an extra weight is added the loads move down visibly (centimetres) and then stops.
The wire is now past its yield point.
This large visible increase in length just after an extra load is added is probably what your teacher was talking about?
Removing the load the wire does not go back to its original length.
Loading the brass wire further produces large visible extensions and then eventually the wire breaks and using a magnifying glass one can easily see the neck which is the characteristic of a ductile fracture.
In your picture the light going through is not in the picture. Since in ideal case al the reflected light is cancelled out, the through going light is the only one which reaches the silicon amplified. Whenever you have cancelling interference in one part, you have amplifying on the other side. If nothing comes out, everything stays in (law of conservation of energy) .
Best Answer
Four years after the OP, this question still doesn't have an answer, yet it comes up in the first page of google searches on fabric stretch, so I will give it a shot.
You are completely correct in your speculation: adding stretchy material to an otherwise non-stretchy fabric does not increase the stretch of the fabric at all. As you suggested, the amount of stretch is completely determined by the length and alignment of the the least stretchy fibers in the weave or the knit.
Instead, stretchiness is increased by making the least-stretchy fibers longer and distributing the extra length in some geometry that allows them to be pulled straighter when tension is applied in the direction of interest. The ability to distribute extra fiber using many different geometries is one of the primary contributions of knitted structures compared to woven ones.
There are two reasons why high-stretch fabrics contain added elastic material. The most important purpose is to improve the recovery of the fabric when the tension is released. The same knitted or woven fabric without the elastomer would stretch just as much, but would be baggy and would recover very slowly, or may not recover until processed in some way (washing, steaming, ironing). Adding elastomer in the right amounts and the right places allows that same fabric to spring back immediately.
The second way in which elastomers improve stretchable fabrics is by adding resistance to stretch. Where a stretchable fabric without elastomers may be stretched with very little force, adding elastomers increases the amount of tension required to stretch the fabric. This allows the fabric to retain its shape and structure until a designed-in amount of tension is applied.